Tracing breaks query plan viewing

If tracing is enabled, then the query plan isn't returned by the gateway.

#### Steps to reproduce

1. Clone `https://github.com/apollographql/federation-demo`;
2. Change `gateway.js#L16` to `const server = new ApolloServer({ schema, executor, tracing: true });`;
3. The query plan doesn't appear anymore on Playground.

##### Other issues

Tracing only shows the request and response ms, it's not displaying per field, query, type and so on =/

Start with gateway.js#L16 in the federation-demo reproduction and compare the query-plan response with tracing disabled and enabled. Trace the gateway path that returns the plan and inspect how tracing changes the request or response; done means Playground still shows the query plan when tracing is enabled, with the intended tracing detail clarified.

Written by the indexing model from the issue text.